Use webpack-4 style hooks.

[mark-raymond]

compiler.plugin is deprecated in webpack 4. This PR uses the new way of hooking into webpack, fixing #32.

Context: I upgraded to webpack 4 today, and got this warning:

(node:8980) DeprecationWarning: Tapable.plugin is deprecated. Use new API on `.hooks` instead
    at InjectByTag.apply (<source path>\node_modules\webpack-auto-inject-version\dist\WebpackAutoInjectVersion.js:3468:29)
    at WebpackAutoInject._executeComponent (<source path>\node_modules\webpack-auto-inject-version\dist\WebpackAutoInjectVersion.js:2879:12)
    at WebpackAutoInject._executeComponent (<source path>\node_modules\webpack-auto-inject-version\dist\WebpackAutoInjectVersion.js:2871:14)
    at WebpackAutoInject._executeWebpackComponents (<source path>\node_modules\webpack-auto-inject-version\dist\WebpackAutoInjectVersion.js:2844:12)
    at WebpackAutoInject.apply (<source path>\node_modules\webpack-auto-inject-version\dist\WebpackAutoInjectVersion.js:2817:12)
    at webpack (<source path>\node_modules\webpack\lib\webpack.js:47:13)
    at Object.<anonymous> (C:\Work\Git\SqlClone\RedGate.SqlClone.Client.Web\tools\build.js:14:1)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Function.Module.runMain (module.js:694:10)
    at startup (bootstrap_node.js:204:16)
    at bootstrap_node.js:625:3

I guess this would make webpack-auto-inject-version incompatible with webpack 3 and earlier; I'm not sure what the best way of dealing with that is.

[bndynet]

@mark-raymond I'm glad to find your PR, but did not work. Maybe I have some package dependencies problems. So I try to write a simple plugin for latest webpack at https://github.com/bndynet/header-injection-webpack-plugin, it worked. Thanks for your PR which gives me some helps.